The present invention relates to a modulating arrangement for phase and amplitude modulation of a wave, the arrangement comprising an input for receiving groups of bits, a phase shifting circuit for producing two phase-shifted components of the said wave and a modulating circuit for modulating the amplitude of the two components as a function of the said groups.
Such arrangements find important applications, specifically in modems, and similar equipment used for data transmission.
Currently, ever higher transmission rates are required whereas the pass-bands of the transmission channels remain limited. A means for harmonising these contradictory requirements consists of using a combined amplitude and phase modulation of a carrier wave. To each amplitude-phase combination there corresponds a group of bits so that for obtaining high rates a large number of combinations has to be used. For example, Recommendation V.33 of the CCITT indicates, for a rate of 14,400 bits per second, the use of 128 combinations, each of which representing a group of 7 bits.
It will be evident that that for making these combinations, memories which are addressed by the said groups and produce the phase and amplitude information signals of the carrier in response thereto, are preprogrammed. However, it is estimated that the size of such a memory, especially for fulfilling the above Recommendation, is too large because this arrangement is a component of an integrated modem. This type of modem uses a signal processor as described in the article by L. MARY "Processeur de signaux: capacite et performances" published in the journal TOUTE L'ELECTRONIQUES No. 527, October 1987, pages 52-60. The processors are often associated with rather small-sized memories on the same chip, so that it is impossible to store many combinations.